Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G. The 3GPP launched a Study Item “Evolved UTRA and UTRAN” (E-UTRA and E-UTRAN). The study will investigate means of achieving major leaps in performance in order to improve service provisioning and reduce user and operator costs.
It is generally assumed that there will be a convergence toward the use of Internet Protocols (IP), and all future services will be carried on top of IP. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain.
The main objectives of the evolution are to further improve service provisioning and reduce user and operator costs as already mentioned.
More specifically, some key performance and capability targets for the long-term evolution are:                Significantly higher data rates compared to HSDPA and HSUPA: envisioned target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink        Improved coverage: high data rates with wide-area coverage        Significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup)        Greater system capacity: threefold capacity compared to current standards.        
One other key requirement of the long-term evolution is to allow for a smooth migration to these technologies.
The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radio access technique to use.
The choice of adaptive multi-layer Orthogonal Frequency Division Multiplexing (AML-OFDM) in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM sub-carriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex is supported by AML-OFDM.
OFDM with Orthogonal Frequency Domain Adaptation
The AML-OFDM-based downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 1 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead, the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms, very large cells, up to and exceeding 120 km cell radius, with large amounts of time dispersion can be handled. In this case, the length is extended by reducing the number of OFDM symbols in a sub-frame.
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (sub-carriers) even if the channel of the whole frequency band is frequency selective due to a multipath environment. Since the sub-carriers experience different channel states, the capacities of the sub-carriers vary and permit a transmission on each sub-carrier with a distinct data-rate. Hence, sub-carrier wise (frequency domain) Link Adaptation (LA) by means of Adaptive Modulation and Coding (AMC) increases the radio efficiency by transmitting different data-rates over the sub-carriers. OFDMA allows multiple users to transmit simultaneously on the different sub-carriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular sub-carrier is very low, it can be assured that sub-carriers are assigned to the users who see good channel gains on the corresponding sub-carriers.
Two different resource allocation methods can be distinguished upon when considering a radio access scheme that distributes available frequency spectrum among different users as in OFDMA. The first allocation mode or “localized mode” tries to benefit fully from frequency scheduling gain by allocating the sub-carriers on which a specific UE experiences the best radio channel conditions. Since this scheduling mode requires associated signalling (resource allocation signalling, measurement reporting in uplink), this mode would be best suited for non-real time, high data rate oriented services. In the localized resource allocation mode a user is allocated continuous blocks of sub-carriers.
The second resource allocation mode or “distributed mode” relies on the frequency diversity effect to achieve transmission robustness by allocating resources that are scattered over time and frequency grid. The fundamental difference with localized mode is that the resource allocation algorithm does not try to allocate the physical resources based on some knowledge on the reception quality at the receiver but select more or less randomly the resource it allocates to a particular UE. This distributed resource allocation method seems to be best suited for real-time services as less associated signalling (no fast measurement reporting, no fast allocation signalling) relative to “localized mode” is required.
The two different resource allocation methods are shown in FIG. 1 for an OFDMA based radio access scheme. As can be seen from the left-hand part of FIG. 1, which depicts the localized transmission mode, the localized mode is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths (time/frequency bins) of a localized signal. On the other hand, as can be seen from the right-hand part of the figure, distributed mode is characterized by the transmitted signal having a non-continuous spectrum that is distributed over more or less the entire system bandwidth (time/frequency bins).
Measurement Reporting
As a common example for uplink measurement reporting we will describe Channel Quality Reporting in this section. As already mentioned above, when allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the sub-carriers into account. Channel quality information (CQI), the control information signalled by the users, allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
CQI is used in a multi-user communication system to report the quality of channel resource(s). Apart from aid in a multi-user scheduler algorithm in the MAC layer on the network side this information may be used to assign channel resources to different users, or to adapt link parameters such as employed modulation scheme, coding rate, or transmit power, so as to exploit the assigned channel resource to its fullest potential.
A channel resource may be defined as a “resource block” as shown in FIG. 2 assuming a multi-carrier communication system, e.g. employing OFDM. In order to have information on the “quality” of this resource block, measurement of the channel quality have to be taken in the receiving side. An exemplary solution for this is to perform a measurement of the Signal-to-Noise-plus-Interference Ratio (SINR) using reference symbols provided by the transmitting side. However, quality reports are not limited to this and could also contain other types of measurement like a Block Error Rate (BLER) or even UE capabilities like decoder complexity or RF improvements. Examples of different CQI compression formats resulting in different CQI reporting types are given in the document “3GPP TSG-RAN WG1 Meeting #46bis, TDoc R1-062808, 09-13 Oct. 2006, Seoul, Korea”. The signalling flow between the network (eNodeB) and the UE for CQI reporting is depicted in FIG. 3.
Assuming that the smallest unit can be assigned or adapted according to the above, in the ideal case CQI for all resource blocks for all users should be always available. However, due to constrained capacity of the feedback channel, this is most likely not feasible. The feedback channel resources available for CQI is limited and these resources have to be shared among all reporting UEs.
Therefore, reduction techniques are required, so as to transmit for example CQI information only for a subset of resource blocks for a given user. One possibility is to report only the strongest resource blocks. Furthermore, different transmission techniques as described in the section above related to OFDM require also different forms of CQI reports. As already described above, FIG. 1 depicts downlink transmissions in distributed and localized mode. Both transmission methods require different CQI reports. The localized mode needs a quality report exactly on the bandwidth fraction used for the transmission to the specific UE, whereas the distributed mode needs information on the whole bandwidth (which would probably be reduced to an average overall value of e.g. SINR due to the resource constraints as discussed above)
Depending on the variability of the channel conditions experienced, the network can decide to configure an UE with different periodicity for CQI reporting. In case of a slowly changing channel, a reduced reporting frequency saves uplink resources on the physical uplink control channel (PUCCH). Intervals are typically in a range of 2 ms to 160 ms and depend on how often channel conditions need to be reported in order to be able to decide on the scheduling as described above. If the networks decides that the reported information is too infrequent or too often, it will reconfigure the corresponding UE with a new reporting periodicity. Thus, the PUCCH parameters are configured by the network individually for each UE that is reporting CQI.
When UEs reporting measurements, e.g. CQI, not only report a single type of report but provide different types of reports in the same allocated resources, this could, allow the network to e.g. make a decision for switching from distributed to localized mode downlink transmission or vice versa. For making such a decision, the network however needs measurement information for both modes. There is therefore a need for a method allowing the network to reliably identify which type of content each measurement report contains. Due to resource constraints on the feedback channel, measurement reports are kept as redundancy free as possible so that it is difficult for the network to detect the measurement report types blindly.